In the prior art, articles of various materials including wood, plastics, glass, inorganic materials, metals, paper and the like are protected with coating compositions comprising UV-curable resins or thermosetting resins. In the case of articles intended for outdoor use, the cured films of coating compositions are exposed to sunlight, which causes the cured films to be gradually degraded so that their properties are exacerbated with the lapse of time. Even the coating compositions on those articles intended for indoor use cannot avoid the problem that the cured film is gradually degraded, because the articles are exposed to light from fluorescent lamps and the like. The cured films of coating compositions encounter the problem of photo-degradation that since sunlight and fluorescent lamp light contains ultraviolet (UV) radiation in a wavelength range of 200 to 400 nm which is detrimental to polymers or the like, the UV radiation causes functional groups in the polymer, residual polymerization catalyst and the like to be excited, whereby photo-degradation reaction takes place. For the purpose of preventing any decline of properties of a cured film by light in the detrimental spectrum, it is a common practice to add benzophenone or benzotriazole base UV absorbers to coating compositions. However, since these UV absorbers are low molecular weight compounds, they are less compatible with polymers. In addition, the absorbers will volatilize or be decomposed during preparation of coating composition, so that little absorber remains in the cured film resulting from coating and curing of the coating composition. Also, the absorber, even if retained in the cured film, will gradually bleed out of the film. For these reasons, the absorber is not fully effective for controlling any decline of properties of a cured film.
Then, JP-A 2001-139924 and JP-A 2002-212491 propose coating compositions comprising a UV-curable resin or thermosetting resin and a UV-absorbing polymer obtained from copolymerization of a UV-absorbing compound having a polymerizable unsaturated group incorporated therein with an addition polymerizable monomer such as a hydrophilic monomer, functional vinyl compound or functionality-free vinyl compound. These proposals intend to overcome the outstanding problem by converting the UV absorbing compound into a higher molecular weight one for thereby improving the compatibility thereof with the polymer component (i.e., UV-curable resin or thermosetting resin) in the coating composition.
The coating compositions of the above-cited patents have the advantage that the retention of the UV-absorbing compound in the cured film is improved due to improved compatibility between the UV-absorbing compound and the curable resin (i.e., thermosetting resin or UV-curable resin). Nevertheless, the inventors found that the adhesion between the cured film (which is cured by UV exposure) and a substrate (or member to be coated) is not sufficient, and some compositions are not fully transparent. Further, where coating compositions are used in electronic parts and building members, the cured films are required to have a high level of mar resistance and wear resistance as well as the hard coat function. If a UV-absorbing compound is added to such a coating composition, the UV-absorbing compound acts against the required properties. There is available no coating composition capable of forming a cured film having a good balance of durability, mar resistance, and wear resistance.